A power supply unit includes a battery, a motor which is driven by the battery, an electric power converting device which is provided between the battery and the motor, a simulated electric leak generating device which generates an electric leak state between the battery and a vehicle body, and an electric leak detecting device which detects a simulated electric leak. A diagnostic system which performs a diagnosis of operating performance of the electric leak detecting device includes an opening/closing device which connects/disconnects the electric power converting device to/from the battery, and an opening/closing detecting device which detects opening/closing of the opening/closing device and which outputs a result of detection to the simulated electric leak generating device and the electric leak detecting device. When opening/closing of the opening/closing device is detected by the opening/closing detecting device, both generation of a simulated electric leak and detection of an electric leak are retried.